Substituent Parameters Impacting Isomer Composition and Optical Properties of Dihydroindolizine Molecular Switches

In an attempt to understand which factors influence constitutional isomer control of 6′- and 8′-substituted dihydroindolizines (DHIs), a series of asymmetric pyridines was condensed with dimethyl spiro­[cycloprop[2]­ene-1,9′-fluorene]-2,3-dicarboxylate. The substituents on the pyridial derivatives ranged from donating to withdrawing and demonstrated control over the isomeric ratios for all DHIs. Substituent control proved to be selective for the highly donating amino, which exclusively formed the 8′ isomer. The same ratios were reproduced via photolytic experiments, which suggested that the condensation reaction is dominated by the product’s thermodynamic stability. The electronic influences of the substituents extends beyond isomer control, as it impacts the DHIs’ optical properties and electrocyclization (switching) rates to the spiro conformers. Our results allow us to predict the syntheses and properties of future 6′- or 8′-substituted DHIs, molecules that will be applied in understanding the role of the dipole vector orientation to work function switching

Spectroscopic Evidence of Work Function Alterations Due to Photoswitchable Monolayers on Gold Surfaces

Taking advantage of surfaces’ response to interfacial dipoles, a class of photochromophores (dihydroindolizine) is demonstrated to alter the work function of the underlying substrate (∼170 meV). This same molecule also provides spectroscopic signatures for correlating the change in molecular structure to the induced change in the surfaces’ electronic properties. Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) allows analysis of the characteristic dihydroindolizine CC (1559 cm^–1) and pyridinium (1643 cm^–1) stretch as a function of photoexcitation. Structural assignments of this photochromophore are corroborated to density function theory calculations. Conformational changes in the monolayers appear in parallel with work function changes and are consistent with both its rate and magnitude

Influence of Defects on the Reactivity of Organic Surfaces

Molecular orientation within organic solids limits the range of applicable surface reactions, with reactive functionalities often recessed and inaccessible to adsorbates. To induce reactivity in heretofore inert orientations of acenes, a defect-mediated mechanism is utilized to functionalize thin-film phase pentacene. This mechanism was demonstrated via correlation of reaction data to defect density, determined via polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) and atomic force microscopy (AFM). By controllably varying the amount of grain boundaries in the acene films, the reaction can be varied from near zero to coverage exceeding a monolayer. The extensive coverage suggests that the reaction propagates from the defects throughout the grains, a prediction borne out via direct observation of reaction progression along the surface from a single dislocation (via scanning electron microscopy). The results support a mechanism whereby the reaction is initialized at the defect sites, especially boundaries of crystal domains, which opens the unfavorable molecular orientation of the (001) pentacene to incoming adsorbates. This exact film configuration and its successful reaction is especially relevant to organic thin-film transistor (OTFT) devices

The Role of Thermal Activation and Molecular Structure on the Reaction of Molecular Surfaces

Though surface modifications of organic thin films dramatically improve optoelectronic device performance, chemistry at organic surfaces presents new challenges that are not seen in conventional inorganic surfaces. This work demonstrates that the subsurface of pentacene remains highly accessible, even to large adsorbates, and that three distinct reaction regimes (surface, subsurface, and bulk) are accessed within the narrow thermal range of 30–75 °C. Progression of this transition is quantitatively measured via polarization modulation infrared reflection absorption spectroscopy, and atomic force microscopy is used to measure the thin-film morphology. Together, they reveal the close relationship between the extent of the reaction and the morphology changes. Finally, the reaction kinetics of the pentacene thin film is measured with a series of adsorbates that have different reactivity and diffusivity in the thin film. The results suggest that reaction kinetics in the thin film is controlled by both the reactivity and the adsorbate diffusivity in the thin-film lattice, which is very different than the traditional solution kinetics that is dominated by the chemical activation barriers. Combined, these experiments guide efforts toward rationally functionalizing the surfaces of organic semiconductors to enable the next generation of flexible devices